Laser diodes (LDs) based on GaN and other Group III-V semiconductors are of great interest because such laser diodes operate in the blue region of the optical spectrum. An edge-emitting GaN LD is typically fabricated by depositing a number of layers of GaN-based semiconductors on a sapphire substrate. The mirrors that form the ends of the laser cavity are created by cleaving the sapphire substrate and GaN layers. Ideally, the cleavage occurs along the crystal planes of the GaN layers leaving smooth facets that are perpendicular to the direction of light propagation in the laser.
In practice, the cleaving is accomplished by cutting the back side of the wafer (i.e., the side opposite to that on which the GaN layers are deposited) with a dicing saw or a scribe to form a groove having a V-shaped cross-section in the sapphire. The depth of the groove is determined by the device used to cut the groove. If a scribe is utilized, the groove is very shallow. If a dicing saw is utilized, the groove may extend a significant distance into the sapphire substrate. The cleaving is performed by applying pressure from the front or back surface of the wafer. Prior art systems that cleave in this manner are described in Unexamined Japanese Patent Publication No. H5-343742 and Unexamined Japanese Patent Publication No. H5-315643.
Unfortunately, sapphire does not cleave easily. As a result, the resulting cleaved face is often rough, and hence, poorly suited for the mirrors that define the ends of the laser cavity.
In addition, the crystal planes of the semiconductor layers are usually not aligned with the crystal planes of the sapphire, so that cleaving the wafer may result in the semiconductor layers having a facet having one or more of the following defects: misalignment with the facet of the sapphire substrate, inadequate flatness, and an undesired orientation.
To overcome these problems, additional processing is often required. For example, the facets are polished in some fabrication systems. In these systems, the cleaved part is mounted on a polishing device and polished with diamond slurry. This method requires that the device be mounted twice in the polishing apparatus, once to polish each of the two mirrors. As a result, polishing contributes substantially to the cost of the devices, and is to be avoided.
Methods based on dry etching of the cleaved device utilizing reactive ion etching (RIE) or reactive ion beam etching (RIBE) have been utilized to smooth the facets without resorting to mechanical polishing. However, these techniques have a number of problems. First, the etch times are significant. Second, etching conditions that provide a smooth surface are not easily obtained. Accordingly, dry etching has not provided the needed solution to the problem of providing mirror quality facets at a cost that does not significantly increase the cost of the final device.
Broadly, it is the object of the present invention to provide an improved method for providing polished facets in Group III-V semiconductor devices.
It is a further object of the present invention to provide a cutting and polishing method capable of obtaining a sufficiently smooth surface without using conventional polishing techniques.
It is a still further object of the present invention to provide a method for fabricating opto-electronic devices that polishes both cut end surfaces of the source material of the opto-electronic device without removing the source material of the opto-electronic device from the polishing fixture.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.